Three Sisters: A special family story of catastrophe

The most iconic image of the Vredefort Dome is of three mountains rearing up at the entrance to the range known as the Dome Bergland. They are the true gateway to the Unesco World Heritage Site.

I call them the Three Sisters, not that the name appears anywhere; it just seems appropriate. We all know of the Three Sisters near the N1 highway in the Karoo, but here, in the hugest visible asteroid crater on Earth, these sisters have a remarkable family story to tell.

Visitors gasp when they learn that the mountains were created in minutes, not the millions of years it usually takes to heave rocks vertically. Mountains they are, but in reality, they are what remains of the severely eroded collar of the Dome core where the asteroid punched into our planet. They are at the root, so to speak, of surface features that disappeared over two billion years.

The sisters appear first in a series of ridges that form the Witwatersrand in this area. They stand around 130 km south of the major Witwatersrand series running through Johannesburg. Why are they here, seemingly out of place?

Look hard and listen to the story unearthed by researchers who tell of a vast inland lake in the bowl of a supercontinent three billion years ago. Forceful rivers flowing off Himalayan-style peaks we call the Limpopo belt spewed gravel and gold powder into deltas that settled down to become the riches of South Africa’s goldfields.

The sisters and their many siblings in the form of rock strata were laid down flat over 300 million years. As sedimentary layers, they slowly dried and solidified into quartzite, a beautiful stone that glimmers with the sheen of ages. They formed the family of the Witwatersrand Supergroup.
Then came a cataclysm. Two billion years ago, a mighty rock from space struck the quartzites, broke them, upended them and drove them deep. The whole Wits series warped and bent so that some poked up near the centre of the impact, and the rest stood at the edges. The inner ring is what we see today as the Dome Bergland; the outer one is the Witwatersrand proper. They are the same rocks.

The Three Sisters were the bottom layer of the first sediments laid down in the ancient lake. They were thrown up, capsized, and are now exposed, lying at an angle backwards. So what you see is the very deepest strata of the Wits series, the flat face of a once horizontal layer now glimmering white in the morning sun.

The sisters are a beautiful, pure marble-like white because, at the time of the rock’s formation, there was no vegetation on Earth to stain the silica (sand) that settled in the lake. They speak for a time when the Earth had no trees or animals, no life except its earliest algae. In due course, those algal blooms would give rise to the oxygen that has sustained complex life on our planet – but that’s another story.
Meanwhile, we can appreciate the serene loveliness of mountains that were blasted into shape by a catastrophe yet stand today as a monument to scientific discovery.

* Graeme Addison is a former professor and writer on popular science, now a national tour guide living in the Dome. You can call or WhatsApp him on 0842452490.